Television apparatus



y 6, 1940. w. A. TOLSON 2,201,839

TELEVISION APPARATUS Filed May 24, 1935 2 Sheets-Sheet l RflD/O RECEIVER P i N VE N TOR William A. ToZ-san Z/W BY ATTORNEY --July 16, 1940.

Filed May 24, 1935 w. A. TOLSON TELEVI S I 0N APPARATUS 2 Sheets-Sheet 2 CUT-OFF \BIHS CUT- OFF INVENTOF? WilliamA.ToZs0n ATTO NEY Patented July 16, 1940 TELEVISION APPARATUS- William A. Tolson, Westmont, N. J., assignor to Radio Corporation of America, a corporation Delaware Application May 24, 1935, Serial No. 23,136

; Claims. (01. 178-75) My invention relates totelevision apparatus and particularly to means for separating synchronizing signals from picture signals at a television receiver. I

5 In British Patent No. 407,409 there is described a television system in which scanning at the receiver is maintained in synchronisin with scanning at the transmitterby transmitting a horizontal synchronizing impulse of short duration at the end of each scanning line and a vertical impulse of the same amplitudeand longer duration at the end of each picture frame. The synchronizing impulses are of greater amplitude than the picture signals and-preferably are in the "black" direction.

In the system described in this British patent I the receiver circuit for separating picture andsynchronizing signals includes a vacuum tube biased substantially to cut-ofi. by means of a biasing battery. The. synchronizing signals are impressed upon the grid of this vacuum tube'in the positive direction whereby they are passed -by the tube, and the picture signals are eliminated. This picture separating circuit has the disadvantage that the bias'on the tube must be adjusted manually for each different amplitude of incoming signal.

An object of my invention is to provide an improved picture signal and synchronizing impulse separating circuit for use in a system of the above-described type 'which does not have the above-mentioned disadvantage.

' A further object of my invention is to provide a receiver circuit which will separate synchronizing impulses from picture signals efl'ectively regardless of variations in the amplitude of the incoming signal. g

A still further object of my invention is to provide television receiver means for maintaining substantially constant the amplitude of synchronizing impulses supplied to cathode ray tube deflecting circuits.

In a preferred embodimentof my invention'I impress the combined picture and synchronizing signals upon a vacuum tube which is provided with what is commonly referred to as grid-leak bias. This bias is obtained by utilizing av grid condenser and a grid resistor having values such that successive positive synchronizing impulses -of a given amplitude impressed upon the grid cause a substantially steady negative bias to be applied thereto, this negative bias being sufiicient to bias the tube beyond cut-oil. In another embodiment of my invention, I provide the vacuum tube with anegative bias by means of a self-bias 'resistor shunted by a' condenser, the self-bias resistor having a much larger resistance value than employed in usual circuits. In both embodiments of my invention, the negative bias increases in response to an 5 increase in the strength 01? an incoming signal.

Other objects, features and advantages of my, invention will appear from the following description taken in connection with the accompanying drawings in which 10" Figure 1 is a circuit diagram of a television receiver embodying my invention;

Figures 2 and 3 are circuit diagrams of other embodiments of my invention; and

Figure 4 is a group of curves which are referred 15 to in explaining my invention. 7

Referring to Figure 1 there is illustrated a television receiver whichis designed to receive picture and synchronizing impulses which have been'transmitted by means of a modulated car- 20 rier wave in accordance with the teachings of the above-mentioned British patent. The receiving apparatus includes a radio receiver l which selects and demodulates the desired carrier wave whereby picture signals and synchro- 25 nizing impulses of the character described above appear in the output circuit thereof.

The picture signals and synchronizing impulses are amplified in resistance coupled amplifiers '2 and 3 and impressed upon the controlgrid 4 of a cathode ray tube 6. The cathode ray tube 6 may be of well known construction comprising an evacuated envelope having therein a cathode "I, the control grid 4, a first anode 8, and

a second anode 9. 'Suitable deflecting means, 35

such as deflecting coils II and I2, are provided for deflecting the cathode ray both horizontally and vertically to effect scanning of a fluorescent screen IE1 at the end of the tube 6.

The horizontal deflecting coils II are supplied with saw-tooth current having a comparatively high frequency from a horizontal deflecting circuit indicated at It. The vertical deflecting coils l2 are supplied with saw-tooth current having a comparatively low frequency from a vertical de- 45 fleeting circuit indicated at It. The deflecting, circuits and I6 may be any of several well known designs, such circuits generally including an oscillator which can be locked in step with synchronizing impulses impressed thereon. 50 The number of amplifier stages in the radio receiver l is so selected that the polarity of the synchronizing impulses impressed upon the control grid 4 of the cathode ray tube is negative whereby the cathode ray is blocked atthe end 55 of each scanning line, this being desirable in order to prevent a trace on the fluorescent screen during the return line period.

The scanning of the fluorescent screen l3 by the cathode ray is maintained in synchronism with the scanning at the transmitter by separating the synchronizing impulses from the picture signal and impressing these impulses upon the deflecting circuits l4 and Hi. In accordance.

with my invention the combined picture signals and synchronizing impulses are taken from a point in the circuit, such as the output circuit of the amplifier 2, where the synchronizing impulses are of positive polarity and are impressed upon the separating circuit enclosed in the dotted rectangle H.

The separating circuit comprises an electric discharge tube I8 which may be of the three element type such as an RCA-56 having a cathode IS, a control grid 2| and a plate 22. The picture signals and synchronizing impulses are fed to the grid 2| through a grid condenser 23. A grid leak resistor 24 is connected between the grid 2| and the cathode IS. A positive potential from any suitable source is applied to the plate 22 through after, only the synchronizing impulses appear in the output circuit of the separating tube l8 and these impulses are fed through a coupling condenser 21 to two filter circuits 28 and 29. The filter circuit 28 includes a resistor 3| of fairly high resistance value connected in series with a condenser 32 having comparatively large capacity whereby only the lower frequency vertical synchronizing impulses appear across the condenser. impulses are supplied through a potentiometer 33 in shunt to the condenser 32 to the vertical deflecting circuit l6.

The filter circuit 29 consists of a small capacity condenser 34 connected in series with a resistor 36 of fairly low resistance value whereby only the comparatively high frequency horizontal synchronizing impulses pass through the con denser 34to develop voltage impulses across the res stor 36 which are impressed upon the horizontal deflecting circuit I4.

Referring now to the operation of the sepa-.

rating tube l8 it will be apparent that since the synchronizing impulses are of positive polarity when impressed upon the grid 2| they will cause a flow of grid current which will charge the grid condenser 23 thereby producing a negative bias on the grid. It will be apparent that the grid 2| and the'cathode I9 function as rectifier electrodes to keep condenser 23 charged The time constant of the circuit including the grid condenser 23 and grid resistor 24 is such that the horizontal synchronizing impulses cause a substantially steady bias to be applied to the grid 2|. The circuit of the separating tube I8 is so adjusted that the biasing voltage applied to the grid 2| is sufllcient to bias the-tube beyond the cut-off point. In order to obtain this adjustment it is generally desirable to apply a voltage to the plate 22 which is considerably less than the plate voltage recommended for normal operation (usually less than half).

The operation will be better understood by referring to Figure 4. The grid volts-plate current characteristic of the tube l8 when a low voltage is applied to the plate 22 is indicated at 31. The composite signal consisting of the picture signals and the synchronizing impulses which are im- Preferably, the vertical synchronizing pressed upon the grid of the tube I8 is represented by the curve 38, the alternating current axis 01' this signal being indicated at 39. It will be seen that the positive synchronizing impulses 38a havecaused a negative bias of the magnitude indicated on the drawings to be applied to the grid 2| of the separating tube, the bias being suflicient to prevent picture signals from producing a flow of plate current. Only the synchronizing signals appear in the output circuit of the separating tube as indicated at 38b. Attention is called to the fact that even though synchronizing impulses of uneven amplitude due to noise or other disturbances are impressed upon the grid of the tube 3, the synchronizing impulses 38b which appear in the output circuit are of substantially the same amplitude. The reason for this is that the grid-cathode impedance of the tube l8 drops to a comparatively low value when the grid is driven positive.

An important feature of my invention is that in the event that the amplitude of the incoming signal changes as a result of fading or as a re: sult of tuning to a different signal, or for any other reason, the bias applied to the grid of the separating tube l8 changes automatically to pro.- vide separation of the picture and synchronizing signals. This action will be understood by. referring to the lower curve 4| in Figure 4. This curve represents the same picture signals and synchronizing impulses represented by the curve 38 but of considerably increased amplitude. The increased amplitude has resultedin the grid 2| being driven farther positive with the result that the biasingcurrent through'the grid resistor 24 has increased to apply a still more negative bias to the grid. Therefore, in Figure 4 the A. C. axis of the picture and synchronizing signals has been moved farther to the left so that the picture signals are still to the left of the cut-off point even though their amplitude has greatly increased. For the reason mentioned above, the

synchronizing signals appearing in the output circuit of thetube l8 will have approximately the same amplitude as before the increase in signalv strength.

The importance of employing a low plate voltage for most conditions of reception will be appreciated by referring to the dotted line characteristic curve 42 shown in Figure. 4. This characteristic curve is for the same tube and circuit conditions as the curve 31 except that ahigher voltage has been applied to the plate 22. It will be noted that the higher plate voltage causes the cut-off point to shift to the left so that the bias which previously caused good separation v of the picture and synchronizing signals represented by the curve 38 no longer is sufficient to produce the desired separation. Under these conditions the only way to obtain the desired separation is to increase the amplitude of the signal impressed upon the grid 2|. Obviously this would usually require the addition of one or more stages of amplification with a resulting like parts are indicated by the same reference numerals. In one particular embodiment of the The same 1 invention the separating tube 43 was of the 227 type and approximately 180 volts was applied to the plate. Instead of applying normal voltage to the screen grid, a low positive voltage of approximately 7 volts was applied. The efiect of utilizing a low voltage on the screen grid of a screen grid tube is the same as that of utilizin a low plate voltage on the plate of a three element tube; Thus it will be seen that the effect of the low screen grid voltage on the separating tube 43 is to make the cut-oif point of the tube less negative with the beneficial results described previously in connection with the curves shown in Figure 4. It will be understood that the operation of the circuit shown in Figure 2 is the same as that of the separating circuit shown in Figure 1.

Instead of obtaining an automatically variable grid bias by means of a flow of grid current, the bias may be obtained by means of the circuit shown in Figure 3. In Figures 1 and 3 like parts are indicated by the same reference numerals. In the circuit shown in Figure 3 the grid bias is obtained by means of a self-bias resistor 44 connected between the cathode and ground, the resistor being shunted by a condenser 46. A grid resistor 41 is connected between the grid and the grounded end of the biasing resistor 44 whereby the flow of plate current through the resistor 44 causes a negative bias to be applied to the grid.

Since this flow of plate current is caused by positive synchronizing impulses being impressed upon the grid 2|, the flow of plate current is intermittent, but because of the condenser 46 a substantially steady negative bias is applied to the grid. The resistance of the self-biasing resistor 44 is made high enough to produce a negative bias which biases the tube I8 beyond the cut-ofl point whereby picture signals are prevented from appearing in the plate circuit just as in the case of separating circuits previously described. The resistance of the self-biasing resistor 44 preferably is of the same order as the resistance of the plate resistor 48 or, in some cases, it may be given a considerably higher value. g

It may be noted that in employing the separating circuit shown in Figure 3, the grid 2| is not ordinarily driven positive since the biasing voltage is produced by the plate current and does not depend upon the flow of grid current. As is the other separating circuits, the biasing applied to the control grid 2| changes automatically in response to a change in the strength of incoming signals.

In the drawings the values oi certain.condensers and resistors have been indicated in microi'arads, ohms, and megohms. It should be understood that these values are given merely by way of example and may be varied within wide limits.

From the foregoing description it will be seen that I have provided separating circuits which separate synchronizing impulses from picture signals regardless of the strength of the incoming signals and without the necessity of a manual adjustment. It will also be seen that I have provided a circuit which supplies synchronizing impulses to the deflecting circuits which impulses are of substantially constant amplitude even though they are of varying amplitude when picked up by the television receiver.

It will be apparent that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

' 1. In a television receiver for receiving picture signals mixed with synchronizing impulses which occur periodically and which are of greater amplitude than said signals, an electric discharge tube having a grid, a plate and a cathode, a grid leak resistor connected between said grid and said cathode, a grid condenser, said grid resistor and grid condenser having such relative values that said condenser holds a charge between successive synchronizing impulses, means for impressing said picture signals and said synchronizing impulses through said condenser upon said grid with said impulses of positive polarity and of suificient amplitude to cause a flow of grid current and thereby bias said tube beyond the cutoff point.

2. The invention according to claim 1 characterized in that a voltage is applied to said plate which is less than half the rated voltage for said tube.

3. The invention according to claim 1 characterized in that said tube includes a screen grid and further characterized in that a voltage is applied to said screen grid which is less than half the rated voltage for said tube,

4. In a television system of the type in which picture signals and synchronizing signals are transmitted as a composite signal and in which at leastthe greater part of the picture signal voltage is of opposite polarity to the synchronizing signal voltage with respect to the alternating current axis of said composite signal, a receiver comprising a cathode-ray tube having a horizontal deflecting device and a vertical deflecting device, a saw-tooth wave generator coupled to said first device, a saw-tooth wave generator coupled to said second device, an electric discharge tube having input electrodes and an output circuit, ineans for coupling said output circuit to at least one of said generators, means for impressing said picture signals and said synchronizing signals upon said input electrodes with said synchronizing signals having a positive polarity, and means including a grid condenser and a grid resistor for biasing said tube beyond the cut-off point.

5. In a circuit in which it is desired to separate positive electrical impulses from the nega-- tive signals in a voltage wave having an alternating current axis, an electric discharge tube having a grid electrode and having an input circuit including a grid condenser and a grid resistor whereby said impulses, when impressed upon the input circuit, cause grid current to flow and charge said condenser, the time constant of said input circuit being such that the charge on said grid condenser holds over a substantial amount between positive impulses, and the ad- Justment of said tube and associated circuit being such that said condenser charge biases said tube beyond cut-oil.

WHLIAM A. TOLSON. 

